core/tools.{h,cpp}: scientific_string_of_double, default precision to 2

bikeshedding e-value outputs
These estimations are by no way more precise than that...

see #4338

algo/core/tools.cpp

@@ -88,10 +88,10 @@ string fixed_string_of_float(float number, int precision)
    return ss.str();
 }
 
-string scientific_string_of_double(double number)
+string scientific_string_of_double(double number, int precision)
 {
    stringstream ss;
-   ss << scientific << number ;
+   ss << scientific << setprecision(precision) << number ;
    return ss.str();
 }
 

algo/core/tools.h

@@ -128,7 +128,7 @@ bool pair_occurrence_sort(pair<T, int> a, pair<T, int> b);
 
 string string_of_int(int number, int pad_to_width=0);
 string fixed_string_of_float(float number, int precision);
-string scientific_string_of_double(double number);
+string scientific_string_of_double(double number, int precision=2);
 string string_of_map(map <string, string> m, const string &before);
 
 /**